Experiments and Considerations Touching Colours (1664)

by Robert Boyle

EXPERIMENT XLIV.

It may (Pyrophilus) ſomewhat contribute towards the ſhewing how much ſome Colours depend upon the leſs or greater mixture, and (as it were,) Contemperation of the Light with ſhades, to obſerve, how that ſometimes the number of Particles, of the ſame Colour, receiv'd into the Pores of a Liquor, or ſwiming up and down in it, do ſeem much to vary the Colour of it. I could here preſent you with particular inſtances to ſhow, how in many (if not moſt) conſiſtent Bodyes, if the Colour be not a Light one, as White, Yellow, or the like, the cloſeneſs of parts in the Pigments makes it look Blackiſh, though when it is diſplay'd and laid on thinly, it will perhaps appear to be either Blew, or Green, or Red. But the Colours of conſiſtent Pigments, not being thoſe which the Preamble of this Experiment has lead you to expect Examples in, I ſhall take the inſtances I am now to give you, rather from Liquors than Dry Bodyes. If then you put a little fair Water into a

cleer and ſlender Vial, (or rather into one of thoſe pipes of Glaſs, which we ſhall by and by mention;) and let fall into it a few drops of a ſtrong Decoction or Infuſion of Cochineel, or (for want of that) of Brazil; you may ſee the tincted drops deſcend like little Clouds into the Liquor; through which, if, by ſhaking the Vial, you diffuſe them, they will turn the water either of a Pinck Colour, or like that which is wont to be made by the waſhing of raw fleſh in fair Water; by dropping a little more of the Decoction, you may heighten the Colour into a fine Red, almoſt like that which ennobles Rubies; by continuing the affuſion, you may bring the Liquor to a kind of a Crimſon, and afterwards to a Dark and Opacous Redneſs, ſomewhat like that of Clotted Blood. And in the paſſage of the Liquor from one of theſe Colours to the other, you may obſerve, if you conſider it attentively, divers other leſs noted Colours belonging to Red, to which it is not eaſie to give Names; eſpecially conſidering how much the proportion of the Decoction to the fair Water, and the ſtrength of that Decoction, together with that of the trajected Light and other Circumſtances, may vary the Phænomena of this Experiment. For the convenienter making whereof, we uſe

inſtead of a Vial, any ſlender Pipe of Glaſs of about a foot or more in length, and about the thickneſs of a mans little finger; For, if leaving one end of this Pipe open, you Seal up the other Hermetically, (or at leaſt ſtop it exquiſitely with a Cork well fitted to it, and over-laid with hard Sealing Wax melted, and rubb'd upon it;) you ſhall have a Glaſs, wherein may be obſerv'd the Variations of the Colours of Liquors much better than in large Vials, and wherein Experiments of this Nature may be well made with very ſmall quantities of Liquor. And if you pleaſe, you may in this Pipe produce variety of Colours in the various parts of the Liquor, and keep them ſwimming upon one another unmix'd for a good while. And ſome have marveil'd to ſee, what variety of Colours we have ſometimes (but I confeſs rather by chance than skill) produc'd in thoſe Glaſſes, by the bare infuſion of Brazil, variouſly diluted with fair Water, and alter'd by the Infuſion of ſeveral Chymical Spirits and other Saline Liquors devoid themſelves of Colour, and when the whole Liquor is reduc'd to an Uniform degree of Colour, I have taken pleaſure to make that very Liquor ſeem to be of Colours gradually differing, by filling with it Glaſſes of a Conical figure, (whether the Glaſs have

its baſis in the ordinary poſition, or turn'd upwards.) And yet you need not Glaſſes of an extraordinary ſhape to ſee an inſtance of what the vari'd mixture of Light and Shadow can do in the diverſifying of the Colour. For if you take but a large round Vial, with a ſomewhat long and ſlender Neck, and filling it with our Red Infuſion of Brazil, hold it againſt the Light, you will diſcern a notable Diſparity betwixt the Colour of that part of the Liquor which is in the Body of the Vial, and that which is more pervious to the Light in the Neck. Nay, I remember, that I once had a Glaſs and a Blew Liquor (conſiſting chiefly (or only, if my memory deceive me not,) of a certain Solution of Verdigreaſe) ſo fitted for my purpoſe, that though in other Glaſſes the Experiment would not ſucceed, yet when that particular Glaſs was fill'd with that Solution, in the Body of the Vial it appear'd of a Lovely Blew, and in the neck, (where the Light did more dilute the Colour,) of a manifeſt Green; and though I ſuſpected there might be ſome latent Yellowneſs in the ſubſtance of the neck of the Glaſs, which might with the Blew compoſe that Green, yet was I not ſatisfi'd my ſelf with my Conjecture, but the thing ſeem'd odd to me, as well as to divers curious

perſons to whom it was ſhown. And I lately had a Broad piece of Glaſs, which being look'd on againſt the Light ſeem'd clear enough, and held from the Light appear'd very lightly diſcolour'd, and yet it was a piece knock'd off from a great lump of Glaſs, to which if we rejoyn'd it, where it had been broken off, the whole Maſs was as green as Graſs. And I have ſeveral times us'd Bottles and ſtopples that were both made (as thoſe, I had them from aſſur'd me) of the very ſame Metall, and yet whilſt the bottle appear'd but inclining towards a Green, the Stopple (by reaſon of its great thickneſs) was of ſo deep a Colour that you would hardly believe they could poſſibly be made of the ſame materials. But to ſatisfie ſome Ingenious Men, on another occaſion, I provided my ſelf of a flat Glaſs (which I yet have by me,) with which if I look againſt the Light with the Broad ſide obverted to the Eye, it appeares like a good ordinary window Glaſs; but if I turn the Edge of it to my Eye, and place my Eye in a convenient poſture in reference to the Light, it may contend for deepneſs of Colour with an Emerald. And this Greeneſs puts me in mind of a certain thickiſh, but not conſiſtent Pigment I have ſometimes made, and can ſhow you when you pleaſe,

which being dropp'd on a piece of White Paper appears, where any quantity of it is fallen, of a ſomewhat Crimſon Colour, but being with ones finger ſpread thinly on the Paper does preſently exhibit a fair Green, which ſeems to proceed only from its diſcloſing its Colour upon the Extenuation of its Depth into Superficies, if the change be not ſomewhat help'd by the Colours degenerating upon one or other of the Accounts formerly mention'd. Let me add, that having made divers Tryals with that Blew ſubſtance, which in Painters ſhops is call'd Litmaſe, we have ſometimes taken Pleaſure to obſerve, that being diſſolv'd in a due proportion of fair Water, the Solution either oppos'd to the Light, or dropp'd upon White paper, did appear of a deep Colour betwixt Crimſon and Purple; and yet that being ſpread very thin on the Paper and ſuffer'd to dry on there, the Paper was wont to appear Stain'd of a Fine Blew. And to ſatisfie my ſelfe, that the diverſity came not from the Paper, which one might ſuſpect capable of inbibing the Liquor, and altering the Colour, I made the Tryal upon a flat piece of purely White Glaſs'd Earth, (which I ſometimes make uſe of about Experiments of Colours) with an Event not unlike the former.

And now I ſpeak of Litmaſs, I will add, that having this very day taken a piece of it, that I had kept by me theſe ſeveral years, to make Tryals about Colours, and having let fall a few drops of the ſtrong Infuſion of it in fair water, into a fine Cryſtal Glaſs, ſhap'd like an inverted Cone, and almoſt full of fair Water, I had now (as formerly) the pleaſure to ſee, and to ſhow others, how theſe few tincted drops variouſly diſperſing themſelves through the Limpid Water, exhibited divers Colours, or varieties of Purple and Crimſon. And when the Corpuſcles of the Pigment ſeem'd to have equally diffus'd themſelves through the whole Liquor, I then by putting two or three drops of Spirit of Salt, firſt made an odd change in the Colour of the Liquor, as well as a viſible commotion among its ſmall parts, and in a ſhort time chang'd it wholly into a very Glorious Yellow, like that of a Topaz. After which if I let fall a few drops of the ſtrong and heavy Solution of Pot-aſhes, whoſe weight would quickly carry it to the ſharp bottome of the Glaſs, there would ſoon appear four very pleaſant and diſtinct Colours; Namely, a Bright, but Dilute Colour at the picked bottome of the Glaſs; a Purple, a little higher; a deep and glorious Crimſon, (which Crimſon

ſeem'd to terminate the operation of the Salt upward) in the confines betwixt the Purple and the Yellow; and an Excellent Yellow, the ſame that before enobled the whole Liquor, reaching from thence to the top of the Glaſs. And if I pleas'd to pour very gently a little Spirit of Sal Armoniack, upon the upper part of this Yellow, there would alſo be a Purple or a Crimſon, or both, generated there, ſo that the unalter'd part of the Yellow Liquor appear'd intercepted betwixt the two Neighbouring Colours.